Well cementing is a process used in penetrating subterranean formations that produce oil and gas. In well cementing, a well bore is drilled while a drilling fluid is circulated through the well bore. The circulation of the drilling fluid is then terminated, and a string of pipe, e.g., casing, is run in the well bore. The drilling fluid in the well bore is conditioned by circulating it downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the well bore. Next, primary cementing is typically performed whereby a slurry of cement in water is placed in the annulus and permitted to set into a hard mass to thereby attach the string of pipe to the walls of the well bore and seal the annulus. By sealing the annulus, migration of reservoir fluids from one zone to another through the annulus is prevented.
Various types of drilling fluids, also known as drilling muds, have been employed in the well cementing process. Oil-based drilling fluids have several advantages compared to water-based drilling fluids such as superior hole stability, especially in shale formations, and excellent lubrication properties. These lubrication properties permit the drilling of well bores having a significant vertical deviation, as is typical of off shore or deep water drilling operations. When a water-based drilling fluid is used to drill a highly deviated well bore, the torque and drag on the casing can undesirably cause the casing that lies against the low side of the well bore to stick. In contrast, oil-based fluids form a thin, slick filter cake that helps prevent the casing from sticking.
Oil-based drilling fluids typically contain some water, making them water-in-oil type emulsions, also known as invert emulsions. The water may arise in the drilling fluid itself or from the well bore, or it may be intentionally added to affect the properties of the drilling fluid. The invert emulsion commonly contains both water-soluble and oil-soluble emulsifiers (i.e., emulsifying agents or surfactants) to stabilize the invert emulsion. Examples of traditional emulsifiers employed in the invert emulsion include polyvalent metal soaps, phosphate esters, fatty acids, and fatty acid soaps. Typically, these emulsifiers impart oil wetting properties to the drilling fluids.
The use of traditional emulsifiers in drilling fluids can complicate the clean up process in open hole completion operations. In particular, oil-based solvents containing surfactants are used to penetrate the filter cake and reverse the wetability of the filter cake particles, thereby converting the oil-wet solids of the filter cake to water-wet solids. Water-wet solids in the filter cake are required so that a subsequent acid wash can be used to destroy or remove the particles. Acid usually cannot be placed in direct contact with a traditional invert emulsion. Otherwise, the direct acid contact would lead to the addition of the acid to the invert emulsion's internal aqueous phase, resulting in a significant increase in the viscosity of the invert emulsion. Cleaning the well bore in this staged manner can be time consuming. Unfortunately, the longer the time required to clean the well bore, the more likely the well bore is to become unstable and collapse. If this occurs, the well bore will have to be re-drilled or opened up before production can occur. Thus, there is a tradeoff between increased production due to a fully cleaned-up well bore and the potential of collapse of the well bore due to instability.
To avoid risking the collapse of the well bore, drilling fluids containing, for example, ethoxylated soya amine emulsifiers, have been developed that provide for a faster clean up of the well bore. Such drilling fluids can be reversibly converted from a water-in-oil type emulsion (i.e., invert emulsion) to an oil-in-water type emulsion that can be easily broken down with an acid soak solution. The invert emulsion is converted to an oil-in-water emulsion by mixing it with an aqueous acid solution that protonates the emulsion. If the subterranean formation produces crude oil, the aqueous acid solution commonly contains a strongly anionic sulfonate surfactant to prevent the formation of aqueous acid solution-crude oil emulsions in the well bore and crude oil sludging therein. However, it has been discovered that due to the presence of the anionic sulfonate surfactant, the emulsifier becomes water insoluble such that the emulsion remains as a water-in-oil emulsion. Further, the aqueous acid solution adds to the internal water phase, resulting in a significant increase in the viscosity of the invert emulsion. The high viscosity emulsion can undesirably seal off the subterranean formation, irreversibly damaging the formation and making oil production impossible.
As such, there continues to be a need for oil-based fluids with improved acid additive compatibility that can be quickly and easily converted from invert emulsions to oil-in-water emulsions without being concerned their viscosity might increase. Using such oil-based fluids would ensure that the subterranean formation penetrated by the well bore does not become plugged. The present invention utilizes an oil-based fluid that may be inverted in a timely manner without risking damage to the formation and that is compatible with typical sulfonate acidizing additives.